Italy has been rocked by its strongest earthquake in decades, just months after tremors killed more than 300 people in the country's central region.

A quake measured at magnitude 6.6 was felt by Italians in the central and southern regions around 7.40 a.m. local time on Sunday, according to the United States Geological Survey (USGS).

Sunday's quake was stronger than the magnitude-6.2 tremor that struck near the town of Amatrice in August, killing hundreds. The two quakes, as well as a magnitude-6.1 tremor on Oct. 26, are part of an "ongoing sequence of damaging earthquakes," the USGS said.

Gregorio Borgia / AP

"This is a complex sequence of related earthquakes occurring on more than one fault segments in central Italy," the USGS said in a statement. "We can expect aftershocks to continue for weeks and possibly months. We cannot rule out the possibility of similar sized or larger events, though the probability of a larger event is low."

The USGS said Sunday's quake was reportedly the largest earthquake in Italy since a magnitude-6.9 quake in 1980.

Sunday's 10-kilometer-deep quake demolished buildings, with tremors also felt in the capital Rome.

Italy's Civil Protection Department said staff were out and checking for damage and injured.

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Firefighters walk down a road in Norci.

Remo Casilli / Reuters

A church on the road to Norcia.

The head of the organisation, Fabrizio Curcio, said: "No deaths have been reported, but there are a number of people injured."

Curcio said 20 people had been injured, none of them seriously. Some 100,000 are estimated to be unable to return home, and some electrical and water services were down, he added.

Curcio said that six reconnaissance airplanes had been deployed to scan the area for damage.

The hilltop town of Amatrice, which was left in ruins in after August's quake, seen Sunday.

Many residents of the region had already left their homes, sleeping in their cars or traveling to the coast, after two earthquakes struck the area last week.

Marco Rinaldi, the mayor of Ussita, one of the small towns affected by the quake, was sleeping in his car when the tremors struck this morning. "Everything collapsed. I can see columns of smoke. It's a disaster, a disaster," he told the Ansa news agency. "I saw hell break out."

Monks from the same church — an international Benedictine community — initially said they were aware of no casualties, but later tweeted that people were stuck in some of the collapsed buildings as rescuers worked to dig them out.

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Pope Francis, speaking during the afternoon prayers in Rome, expressed his "closeness" to the victims of the earthquake. "I pray for the injured and for the families that have suffered major damage, as well as for the personnel involved in rescue and assistance," he told crowds from a window in St. Peter’s Square.

The Vatican's St. Paul Outside the Walls basilica was closed for a few hours Sunday morning after some plaster fell, sparking fears the historic building had been damaged in the quake. The basilica was later reopened after Vatican firefighters inspected the structure for damage.

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